01 GT to 03 Terminator Swap

If you are really serious about solving these problems there are two things that will prove vital.

• An ODB2 scanner. One with graphing is even better.
• A full set of Ford wiring diagrams.

Here's why an ODB2 scanner is so important. In the 1999+ model year the transmission OSS speed sensor output signal goes directly to the PCM. The PCM then "tells" the cluster what speed to display via an electronic SCP LAN message. So the first step to trouble shoot a speedometer problem is to know "does the PCM know the car's speed?". For that an ODB2 scanner is needed. The next step depends upon the results of the first question.

If looking for an educated guess I would lean towards a problem in the transmission OSS speed sensor where the PCM is not getting a valid speed signal. This would also explain why the shift light is acting strange. Obviously if the PCM doesn't know the car's speed then the PCM can't tell the cluster what to display.

Here's some information on an affordable Windows based unit.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

The next thing is a full set of Ford wiring diagrams for the 2001 model year and the 2003 model year. The first thing to find out is "how" does the dash boost gauge work? Does it use a separate wire or is it done via SCP message? If done via SCP message, then does the PCM "know" what the boost is.

If interested in getting a full set of Ford service manuals and wiring diagrams on data DVD I may be able to help. PM if interested.